In recent years, along with the progress in the portability and codeless tendency of electronic instrument, a demand for a non-aqueous electrolyte secondary battery which is small in size and light in weight and has a high capacity and high energy density, has been increasingly high. As a cathode active material for the non-aqueous electrolyte secondary battery, a composite oxide of lithium and a transition metal, such as LiCoO2, LiNiO2, LiNi0.8Co0.2O2, LiMn2O4 or LiMnO2, has been known.
Particularly, a lithium secondary battery using lithium cobalt composite oxide (LiCoO2) as a cathode active material and using a lithium alloy, graphite, carbon fiber, a silicon alloy or the like as a negative electrode, is widely used as a battery having a high energy density, since a high voltage at a level of 4 V can thereby be obtainable.
However, for a non-aqueous electrolyte secondary battery for consumer application, further improvement in the discharge capacity per unit volume of the positive electrode plate (which may be referred to as a volume capacity density in this specification), i.e. higher energy densification, is desired.
In order to obtain a lithium ion secondary battery having a high energy density, the positive electrode is required to have a high electrode density. In order to improve the electrode density, the tap density or pressed density is regarded as a proper index, and especially, the pressed density is regarded as highly interrelated with the electrode density.
With respect to such packing properties, the following various studies have been made. For example, in Patent Document 1, it is stated that factors influential to the tap density or pressed density of a cathode active material, are the shapes of particles, and the sizes and particle size distribution of primary particles. Further, it is disclosed that the average particle size of LiCoO2 to be used for industrial application is from 1 to 20 μm, and the larger the average particle size, the higher the pressed density.
Patent Documents 2 to 5 disclose a mixed powder having a wide particle size distribution, wherein a powder composed of particles having large particle sizes and a powder composed of particles having small particle sizes are mixed. Further, it is disclosed that by using such a mixed powder, it is possible to improve the packing properties, since small particles will enter into spaces among large particles. Particularly, it is disclosed that it is possible to obtain a cathode active material having high packing properties by mixing two types of active material powders which are substantially different in the particle sizes.
Further, Patent Document 6 discloses that it is possible to obtain a cathode active material which has a high electrode density and is excellent in the slurry and electrode coating uniformity and which has good electrochemical properties, by regulating the particle size distribution of a powder of the cathode active material and by using a powder having a specific sharp particle size distribution where particles of the powder are not excessively large and not excessively small.
Furthermore, Patent Documents 7 to 9 disclose that by using a powder having a specific crushing strength as a powder of a cathode active material, or controlling the state of its presence in the powder layer, it is possible to improve the packing properties of the powder when a compressive stress is exerted to the powder.